An attached matter detector includes a light source configured to irradiate light toward a plate-like transparent member from one surface thereof, an imaging device configured to image reflection light by attached matter on a surface of the plate-like transparent member, a memory configured to previously store flare information obtained by the image device by turning on the light source in a state without having disturbance light and the attached matter on the surface of the plate-like transparent member, and a difference information-obtaining device configured to obtain difference information between a light-up image as an image obtained by the imaging device with the turned-on light source and a light-out image as an image obtained by the imaging device with the turned-off light source, wherein the attached matter on the surface of the plate-like transparent member is detected based on the difference information and the flare information stored in the memory.